1. Field of the Invention
The invention relates to a method of fabricating composite structural components from layers of solid metallic materials and frothable metallic materials.
2. The Prior Art
Light structural components such as, for instance, hollow bodies, provided with two cover layers made of a solid metallic material and a layer of metallic foam placed between them are known. Being of light weight these structural components are characterized by a particularly high rigidity and strength. From German patent specification DE 41 01 630 it is known to fill hollow bodies with frothing aluminum and that the frothing process is carried out in the interior of the hollow body. This results in a particularly light structural component. The connection between the foam filler and the hollow body may be impeded by the presence of oxide layers on all surfaces of the hollow body and on the surface of the expanding foam.
Sandwich-like composite materials may also be fabricated by adhesively affixing cover layers of solid metallic material to bodies of foamed aluminum. These, however, suffer from the lack of strength of the adhesives, especially at high temperatures. Composite materials are known from German patent specification 44 26 627 which are fabricated by roll coating frothable aluminum with liners of solid metallic material such as, for instance, steel, aluminum or titanium, followed by frothing of the frothable aluminum. Hollow bodies made by this process are provided at their exterior and at their interior with a cover layer of solid metallic material. The internal cover layer contributes little, however, to the rigidity and strength of the component; yet it does increase the total weight of the component.
It is the task of the present invention to provide a structural component which is of high rigidity and strength, but of low weight. Another task of the invention resides in a method of fabricating such a component.
The task is accomplished by the method hereinafter described in greater detail.
In accordance with the invention, a structural component is provided which is provided with a layer of solid metallic material and of which at least one surface is formed by a frothable metallic substance. This surface may be either the external or the internal surface. The component may be structured as a planar composite sheet or as a hollow body. The method of fabricating this component insures an excellent connection between the solid metallic cover layer and the frothable layer and a strong bond between the frothable layer and the cover layer.
A component formed as a planar composite sheet may consist of a solid metallic layer and a layer of a frothable metallic substance strongly adhering thereto. Such composite sheets are particularly suitable as side impact protection bars of automotive passenger vehicle doors. Such a side impact protection bar thus consists of a hollow profile on the interior surface of which there is provided a strongly adhering metallic foam which while significantly contributing to the rigidity and strength of the component contributes but little to its weight.
In another structure of the composite sheets the component consists of several layers of a solid substance and intermediate layers of a frothable metallic substance wherein both or only one surface of a composite sheet structured in this manner may be formed of a layer of frothable metallic material. In another embodiment of the invention, a composite sheet is provided with a layer solid metallic material positioned between two frothable layers. Such a composite sheet exhibits an especially high rigidity and strength at a very low weight.
The structural components in accordance with the invention with an asymmetric layer sequence are preferably fabricated by pack rolling. This involves alternatingly arranging layers of a solid metallic material and layers of frothable metallic material in superposition. The invention also provides for using loose powder mixtures containing a blowing agent or precompacted (e.g. by cold isostatic pressing) powder mixtures instead of the finished frothable layers. In such cases one or more connected, continuous non-porous separating foils are used as release agents in view of the fact that otherwise emulsions may become intermingled with the loose or precompacted powder mixture. In such cases the loose or precompacted powder mixtures are not condensed to frothable layers before the cladding and laminating process. Fusion during the roll process between the frothable layers which are part of individual composite sheets is prevented by the provision of a release agent such as, e.g. an emulsion of graphite or boron nitrite, or by a separating foil, between the layers. A foil is used as the release agent where the frothable layer is a powder mixture containing a blowing agent. The packets thus formed are then subjected to a rolling process. The result of the rolling process are components with frothable layers on their surfaces, the components being separated by the release agents between the frothable layers.
Provided they result in a good bond between the frothable metal layer and the cover layer of solid metallic material, other known cladding and laminating processes may also be utilized. They may be drawing, thermal extrusion, explosive cladding, coextrusion, extrusion cladding or HIP cladding processes.
The quality of the bond of composite materials fabricated by the method in accordance with the invention is such that the planar composite material sheets may be cold-formed or further processed into hollows with longitudinal welding seams. To fabricate irregularly shaped hollow components such as, e.g. tanks and transverse links, the planar composite sheets may first be shaped by any desired forming processes such as pressing, deep drawing or cavity pressure forming, before finishing the desired hollow body.